This invention relates generally to low thermal mass boilers and more particularly to a wound boiler having an improved free standing, removable and replaceable combustion chamber therein.
In U.S. Pat. No. 4,261,299 a wound boiler is shown having a centrally disposed generally elongated fire box defined by the innermost heat exchanger convolutions of a heat exchanger slab which is wound upon itself around the longitudinal axis for the boiler in a plurality of heat exchanger convolutions. The innermost heat exchanger convolution and the next adjacent heat exchanger convolution of the slab being disposed in spaced relation to each other so as to define an exit at the lower section of the fire box through which hot gaseous combustion products exit and which provides communication with the spaced passages formed between the convolutions to conduct the products of combustion from the fire box to an expansion space and its associate exhaust stack or flue for the wound boiler to provide means for passing waste combustion products therefrom.
In U.S. Pat. No. 4,261,299 the combustion chamber or fire box is preferably lined with a refractory material to improve combustion therein or the fire box is sized to permit better combustion without a refractory liner.
U.S. Pat. No. 4,261,299 follows conventional wet base boiler arrangements wherein the radiation from the flame to the boiler heat exchanger surface is used to enhance heat transfer. However, instead of passing the combustion gases through a port in the top of the combustion chamber or fire box to in turn communicate with the exhaust stack, this patent discharged the gaseous products of combustion out of an exit port in the lower section of the fire box in communication with another heat exchanger flow passage before the waste combustion products are passed to the exhaust stack or flue. This improved residence time for the flame in the center of the fire box and heat transfer with the convoluted heat exchanger surfaces. It was found that the operatively associated convoluted combustion gas passages of the wound boiler reduced the exhaust stack temperature of the waste combustion products significantly thus increasing the efficiency of the wound boiler.
However, the exit port in the lower section of the combustion chamber or fire box in U.S. Pat. No. 4,261,299 caused problems in that unburned hydrocarbons, non-condensible gases such as nitrogen and CO.sub.2 from the burning process and droplets of unburned hydrocarbons collected at the top of and on the walls of the firebox would inhibit combustion more particularly on cold start-ups.
The direct application of refractory materials to the adjacent heat transfer surfaces defining the combustion chamber or fire box as shown in U.S. Pat. No. 4,261,299 severely impeded heat transfer. It however improved flame propogation by radiation and reduced impingement but caused stack temperatures to increase dramatically.
In the present invention these problems are overcome by inserting a free standing heavily insulated generally elongated hollow cylindrical member within the space defined by the innermost convolution of the wound slab so that it lies in spaced relation to the innermost heat exchanger convolution and forms or defines a combustion chamber or fire box therein and an extra or additional inner substantially annular elongated combustion products flow passage between the outer surface thereof and the wall of the innermost heat exchanger convolution. The combustion chamber formed in the hollow cylindrical member is sized and shaped with a volumetric capacity to permit completion of combustion therein and is further provided with a slotted opening at the top section remote from the fuel inlet end of the combustion chamber which slotted opening coacts with a turning guide on the cylinder member adjacent thereto to direct and turn the gaseous products of combustion so they pass from the slotted opening in a reverse flow direction through the inner annular elongated combustion products flow passage, initially, parallel to the longitudinal axis of the wound boiler and then radially downward about the outside or exterior surface of the hollow cylindrical member to exit through the exit port formed between the inner most heat exchanger convolution and the next adjacent heat exchanger convolution formed by the wound horizontal heat exchanger slab. The combustion products then flow to an outer combustion product flow passage which in turn communicates with the expanded gas passage and the exhaust stack in the same manner as above described for U.S. Pat. No. 4,261,299.
This construction acts to eliminate dead spots at the top section of the combustion chamber because the flow pattern through a top exhaust is more normal for hot gaseous combustion products. The extended flow pattern for the hot gaseous combustion products acts to permit completion of combustion thus reducing the problem of collected unburned hydrocarbons and non-condensible gases and the peripheral gas flow around the sides of the outer surface of the cylindrical member acts to improve heat transfer to produce or further significantly drop in the exhaust stack temperatures of approximately 50.degree. F. as compared to an equivalent design with the same size heat transfer surfaces.
While this construction reduced radiation from the flame which is propogated and confined within the combustion chamber defined by the hollow cylindrical member which would normally decrease heat transfer, it has been found that an improvement in heat transfer efficiency has been obtained possibly due to improved velocity control and due to the increased flow path length of the hot combustion gases as they pass about the outside of the cylindrical member through the additional inner combustion gas flow passage formed between the outer surface of the combined combustion chamber and liner member and the inner wall of the innermost heat exchanger convolution of the heat exchanger slab of the wound boiler.